1. Field of the Invention
The present invention relates to data storage devices and, more particularly, to analog storage devices using transistors of a floating gate electrode structure based on tunnel effect.
2. Description of the Related Art
Neural network systems have wide applicability in various technical fields including automatic recognition systems, such as character recognition systems, voice recognition systems, etc., activation control systems for robots and neuro computer systems incorporating artificial intelligence, and studies thereof have become more and more active recently. In the neural network systems, a storage device, which is capable of temporarily or permanently storing analog information handled in the systems as it is, plays a very important role. This type of storage device is referred to as the "analog memory."
A presently available electrically erasable programmable read only memory (EEPROM) has been employed as such an analog memory. The EEPROM includes an array of memory cells arranged in a matrix of rows and columns. Each cell of the EEPROM has a floating gate tunneling metal oxide semiconductor (FATMOS) field effect transistor. An arbitrary memory cell is individually addressable by applying electric signals to a given row and a given column of the memory array.
Typically, analog information or data to be stored is converted to a voltage signal in pulse form which is modulated in the number of pulses (or pulse width) in accordance with a pulse density modulation system. The storage of such a pulse signal in the EEPROM is performed as follows. Between the control gate electrode of a selected memory cell transistor and one of its current-carrying electrodes is externally applied a suitable electric field for a period of time proportional to the number of modulated pulses (or pulse width) of the signal. The tunnel effect allows a corresponding amount of minority carriers (electrons or holes) to flow into the floating gate electrode, thereby charging the floating gate electrode. Thus, the analog data is stored in (written into) the selected memory cell transistor. This is referred to as the "tunnel injection writing" technique.
A significant problem with the analog storage system using the tunnel injection writing technique is that it is very difficult to accurately charge the floating gate electrode of a memory cell transistor with an amount of electric carriers proportional to analog information to be stored. This difficulty depends mainly on the fact that the amount of minority carriers injected into the floating gate electrode is not a linear function of the field application time. In other words, even if the strength of an externally applied field is held constant, the amount of tunneled carriers to the floating gate will vary exponentially with respect to the field application time.
The nonlinearity of the amount of tunneled carriers is considered to result from the following reasons. An unintended electric field is produced internally around the floating gate electrode when the storage of tunneled carriers in a selected memory cell transistor is in progress in the EEPROM. This unintended electric field is inevitably created by the carriers stored on the floating gate electrode. The field behaves in such a way as to cancel out the internal field produced between the floating gate electrode and the substrate by the external electric field. The internal field of the cell transistor can no longer maintain a desired strength corresponding to stored data. The strength of the internal field will decrease gradually. As a result, even if the external field is constant, the amount of stored carriers per unit time will gradually decrease with time. The presence of the nonlinearity will delay practical application of the tunnel injection writing technique to analog storage devices. For this reason, an early solution to this problem is desired ardently among those skilled in the art.